Infrared intrusion sensor

ABSTRACT

An infrared intrusion sensor is provided to detect a human target anywhere in a 20 X 20 foot area. A wide angle portraittype lens of germanium is utilized to provide reasonably uniform imaging over the field of view of approximately 70* X 70*, while still focusing on a flat image plane. This allows the use of a flat, extended thermopile detector having all active junctions which are arranged in columns to cover the desired area, and connected so that the columns alternate in a positivenegative configuration. The detector junctions have different sizes and spacing, with the junction concentration being roughly inversely proportional to the square of the range.

United States Paten Leftwich et al. t

[in 3,792,275 [45] Feb. 12, 1974 [75] Inventors: Richard F. Leftwich,Pound Ridge,- N.Y.; Robert T. Ensor, Redding, Conn.

[73] Assignee: Barnes Engineering Company,

Stamford, Conn.

22 Filed: Dec. 26, 1972 [21] Appl. No.: 317,983

[52] US. Cl 250/338, 250/342, 250/344 [51] Int. Cl. G0lt 1/16 [58] Fieldof Search 250/344, 342, 338, 345; 350/1; 340/227, 278; 73/359, 355 R,341

[56] References Cited UNITED STATES PATENTS 2,392,873 l/1946 Zahl250/342 2,198,725 4/1940 Smith 1 250/338 2,357,193 8/1944 Harrison073/355 R INFRARED INTRUSION SENSOR Quereau 73/355 R Evans 340/228 RPrimary ExaminerHarold A Dixon Attorney, Agent, or Firm-Joseph Levinson5 7 ABSTRACT tion concentration being roughly inversely proportional tothe square of the range.

7 Claims, 5 Drawing Figures PAIENIEBFEB 1 2 1914' SHEET 1 M Q ELECTRONICALARM LOGIC BFL MM FIGURE 2 PAIENIEB FEB 1 21924 APERTURE AREA (mm 6 O Ol l I l :4 2| FIELD ANGLE (DEGREES) PAIENIEDFEB 21914 3.792275 sum 3 ora PAIENIED FEB 1 21974 SHEET M? d Lu LU u.

FLOOR FEET 2O IIIIIIIIIIIlllll lllllllllII ZONE llCII 27 ZONE "B" 1 ZONE"A" 25" INFRARED INTRUSION SENSOR BACKGROUND OF THE INVENTION Thisinvention relates to an infrared intrusion sensor and more particularlyto an infrared system employing a simple single lens having a highrefractive index for providing reasonably uniform imaging of a widefield of view on a flat thermopile detector configuration to adequatelycover the field of view.

Radiation generated by the intruder is collected and picked up by asuitable detector andprocessed to pro vide an alarm. A number of suchsystems have been proposed or used, in which radiation from the field ofview is imaged on a small detector utilizing a multifaceted mirror orthe equivalent thereof. This approach limits the collecting aperture,since only one facet is collecting energy from a particular direction. Acompromise must be made between number of facets and collecting apertureor sensitivity.

The detector in such a system, which in effect has radiation appliedthereto by individual mirrored elements, sees only the point in thefield of view imaged by that mirrored segment, making the imaging poorand limiting the number of elements which can be used, and cutting downthe sensitivity of the system.

In an application, Ser. No. 209,660, entitled Intrusion Detector, whichis assigned to the assignee of the present invention, a wide field ofview intrusion detector is provided by a single optical collector. Theradiation collected thereby is applied to a thermopile detector whichhas a plurality of rows of thermocouples having all active junctionswith alternate polarity, which are aligned in columns of the samepolarity. The system responds to an object moving thereacross to producean output signal of alternately changing polarity. The present inventionis directed to improvements in this type of system, and particularly tothe optical element in combination with the detector which forms thesensor.

SUMMARY OF THE INVENTION It is an object of this invention to provide animproved infrared intrusion detector with high sensitivity over a widefield of view.

A further object of this invention is to provide an improved infraredintrusion sensor which is capable of detecting a human target anywherein a 20 X 20 feet area utilizing a single lens and detectorconfiguration.

In carrying out this invention in one illustrative embodiment thereof, awide-angle, portrait-type lens having a high refractive index isutilized to provide reasonably uniform imaging over a wide field of viewwhile focusing on a flat image plane. A flat extended thermopiledetector is positioned on that flat image plane. The

thermopile detector has a plurality of all-active junction thermocouplesarranged in columns to fill the area covered, which thermocouples areconnected to be alternately positive and negative-going. Thethermocouple detector elements have a different spacing and size, withthe greatest density and smallest elements being located where thegreatest range is encountered and the greatest sensitivity required.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an optical schematic andelectrical block diagram of an intrusion sensor system embodied in thisinvention.

FIG. 2 is a graph of the aperture area versus focus for various anglesof the field of view of the lens shown in FIG. 1.

FIG. 3 is a graph of aperture area versus field angle for the lens shownin FIG. 1.

FIG. 4 shows the top view of the detector employed in the sensor shownin FIG. 1.

FIG. 5 is the diagram of the area which must be covered by the detectorshown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT The problem solved by thepresent invention is to provide a high-sensitivity infrared sensor todetect a human target anywhere in a 20 X 20 feet room where the sensoris mounted one foot below an 8 feet or 9 feet ceiling. This requires afield of approximately 70 on the X-Y axis, which will be somewhatsmaller in the corners. The optics of the sensor must be able to focus ahuman target anywhere in the aforesaid field on a small, sensitiveinfrared detector. This is accomplished as shown diagrammatically inFIG. I. A special wideangle portrait-type lens 15 is provided with anaperture provided by the aperture stop 16. The lens 15 focuses on a flatimage plane occupied by an extended flat detector 20, preferably in theform of an evaporated thermopile. Signals from the detector 20 areapplied to electronic logic 22 which provides the function of amplifyingand discriminating against unwanted signals, and applying the desiredsignals to the alarm 24. Various forms of logic alarms may be provided,such as those shown and described in the aforesaid application, and arenot considered part of the present invention.

The lens 15 is used in an unconventional manner with a piano frontsurface 12, thus with an infinite radius forward, which maintainsrelatively uniform aberrations over the entire field of view at theexpense of optimum imagery in the center only. The back surface 14 ofthe lens 15 is convex and brings the image of the field of view to afocus on the flat extended detector 20. To achieve high performance witha single lens element 15, a high index of refraction is required, andthe lens is preferably made of germanium whichhas a refractive index of4. The material of the lens 15 must uniformly image the infraredwavelengths normally emitted from the human body which are largely inthe 8-14 micron region, which makes germanium an ideal selection. Thegermanium lens is provided with a 4-micron cut'on coating, therebyrejecting any signals from such sources as tungsten lamps and sunlightto help prevent false alarms. The cut-on coating also acts as anantireflection coating.

An important compromise was made in the lens 15 design to achievemaximum sensitivity at the greatest range, which would be about 30' atthe opposite room corner. The optical axis 13 of the lens 15 is arrangedto approximate this maximum direction. Ofl'-axis, the distance isnecessarily smaller, never exceeding 20' at the greatest angle. Sincethe signal from a human target is inversely proportional to the distancesquared, the effective collecting area off-axis can be reduced by (20/30feet) or approximately 2:]. This was done to maximize sensitivityon-axis. The aperture area vs focus for a plurality of angles is shownin FIG. 2. The effective collecting or aperture area vs angle is shownin FIG. 3. The latter is shown for a 0.3 mm width, which is atypicalbest size for a sensitive thermopile detector junction. The lens focallength is selected to image a 6 inches wide human face on this width.The lens fnumber was then speeded up to better than 1709 to achievemaximum collecting area on-axis as described above. Merely as anillustrative example of one embodiment of this invention, the aperturestop 16 is 22 mm in diameter, the radius of convex surface 14 of thelens 15 is 54.8 mm, the distance I from the aperture to the plano frontsurface 12 of the lens 15 being 12 i1 mm, and the distance trepresenting the thickness of the lens is 2.5 :0.2 mm.

The preferable form of infrared detector for use in the presentapplication is a thermopile detector. As will be seen in FIG. 4, thethermopile detector is mounted on a base 26 and the thermopile is formedof a series of thermocouples of dissimilar metals 32 and 31 formingjunctions therebetween, which is blackened to enhance the response. Itshould be appreciated that the thermopile as it appears on the drawingis considerably out of scale, and is shown in diagrammatic formfor easeof illustration. The metals 32 and 31 may be bismuth and antimony, orany other suitable thermocouple material. The thermocouple junctions 30are formed on a substrate 28 of a thin insulating material, such aspolyethylene terephthalate or other suitable material which is a goodinsulator and lends itself to deposit of metal thereon by evaporationtechniques. The construction of the thermopile 20 is formed usingconventional vacuum evaporation techniques on the substrate 28, and itsconstruction differs from conventional thermopile construction in thatall of the junctions are active, there being no reference junctions.With this construction, alternating polarities are set up across the rowof thermocouples, and the thermopile 20 is arranged in IQ columns, onethrough 10, with each thermocouple in its respective column being of thesame polarity. All of the thermocouple junctions 30 are seriallyconnected between output terminals 34 and 38 with the rows beinginterconnected serially by the connector pad 33. The terminal pads 34and 38 are connected via leads and 40 to feed-through pins 36 and 42,respectively. With this type of construction, an object moving acrosscolumns 1-10 will generate alternate positive and negative pulses acrossthe output leads 35 and 40, which signals can be processed to detect thepresence of an intruder.

The exact detector pattern to cover the desired area is determined withcare. FIG. 5 shows the basis of that selection in which the 20 X 20 feetarea is divided into zones A, B, and C covering the desired 70. Zone Crepresents the farthest distance at the center, or the diagonal acrossthe room, Zone B being the intermediate zone, and Zone A being directlyunder the sensor. As will be apparent from FIG. 5, the sensor looks outin a fan-shaped span and the detector must be designed to cover theentire area. To this end, and as will be seen in FIG. 4, l0 columns 1-10are provided to avoid leaving open areas in the room. The columns areconnected to be alternately positive and negative going so thatalternate positive and negative signals are produced when an intruderproceeds across the columns. The greatest density and the smallestelements are placed where the greatest range is encountered and thegreatest sensitivity needed, thus in Zone C. However, as the close rangeregion is approached, underneath the optical head, the elements arelarger and wider spaced. Also it should be noted that the cornerjunctions which are not needed to cover the room are eliminated. In thisway, signal-to-noise ratio is maximized,- since all junctions areconnected in series, and fewer junctions mean less resistance andtherefore less noise. Additionally, the junctions 30, and thus thesensitivity, are concentrated in the region Zone C, where they areneeded most.

Merely by way of example, and illustrative of one embodiment, thefollowing dimensions are given:

Zone A 28 junctions, 0.3 mm width X 0.4 mm

height.

Zone B 59 junctions, 0.3 mm Width X 0.3 mm height.

Zone C 211 junctions, 0.3 mm width X 0.2 mm height.

Row separation in Zone A 1.48 mm; Zone B 0.9 mm; Zone C 0.352 mm.

Total distance between rows 20.9 mm.

Distance between columns 1 and 10 20.7 mm.

The use of the evaporated thermopile is beneficial because it has nopower drain and is stable under DC conditions. It lends itself inconstruction due to the simplicity of evaporating large complicatedextended patterns, and lends itself to ease of positive-negative hookupwhen used in the all-active-junction mode. The thermopiles response isindependent of ambient temperature, and also has excellent sensitivityto the infrared radiation generated by the human desired to be detected.With respect to construction, the entire pattern may be deposited usingsuitable masks on a flat surface by evaporation techniques. The use of alarge field being covered by a single lens plus an extended flatdetector provides greater coverage and better sensitivity than thosesystems using multifaceted mirrors and a single detector.

Since other modifications varied to fit particular operatingrequirements and environments will be apparent to those skilled in theart, this invention is not considered limited to the examples chosen forpurposes of illustration, and covers all changes and modifications whichdo not constitute departures from the true spirit and scope of thisinvention.

We claim:

1. An infrared intrusion sensor for providing high sensitivity coveringa wide field of view, comprising a. a wide angle lens having a highrefractive index for providing reasonably uniform imaging over a widefield of view. and being focused on a flat image plane,

b. an extended flat infrared detector located on said flat image plane,

c. said detector having a plurality of all active detector elementsarranged in columns to fill the area covered which are connected to bealternately positiveand negative-going on being activated by anintruder,

' (I. said detector elements having a different spacing and size withthe greatest density and smallest elements being located where thegreatest range is encountered and the greatest sensitivity required.

2. The infrared intrusion sensor set forth in claim 1 in which said wideangle lens is germanium.

3. The infrared intrusion sensor set forth in claim 1 wherein said lenshas a plane front surface and a con vex back surface.

4. The infrared intrusion sensor set forth in claim 1 wherein saidinfrared detector is a thermopile comprised of all active junctionthermocouples.

5. The infrared intrusion sensor set forth in claim 4 wherein said allactive junction thermocouples are serially interconnected.

6. The infrared intrusion sensor set forth in claim 1 wherein the areato be covered is divided into three zones with the close zone underneaththe lens having detector elements which are larger and wider spaced,with size and spacing being reduced and the number of elements increasedin the next immediate zone. and the spacing and size reduced further andthe number of elements being substantially increased in the zonefarthest distance from said lens.

7. The infrared intrusion sensor set forth in claim 1 wherein theconcentration of detector elements is roughly inversely proportional tothe square of the

1. An infrared intrusion sensor for providing high sensitivity coveringa wide field of view, comprising a. a wide angle lens having a highrefractive index for providing reasonably uniform imaging over a widefield of view and being focused on a flat image plane, b. an extendedflat infrared detector located on said flat image plane, c. saiddetector having a plurality of all active detector elements arranged incolumns to fill the area covered which are connected to be alternatelypositive- and negative-going on being activated by an intruder, d. saiddetector elements having a different spacing and size with the greatestdensity and smallest elements being located where the greatest range isencountered and the greatest sensitivity required.
 2. The infraredintrusion sensor set forth in claim 1 in which said wide angle lens isgermanium.
 3. The infrared intrusion sensor set forth in claim 1 whereinsaid lens has a plane front surface and a convex back surface.
 4. Theinfrared intrusion sensor set forth in claim 1 wherein said infrareddetector is a thermopile comprised of all active junction thermocouples.5. The infrared intrusion sensor set forth in claim 4 wherein said allactive junction thermocouples are serially interconnected.
 6. Theinfrared intrusion sensor set forth in claim 1 wherein the area to becovered is divided into three zones with the close zone underneath thelens having detector elements which are larger and wider spaced, withsize and spacing being reduced and the number of elements increased inthe next immediate zone, and the spacing and size reduced further andthe number of elements being substantially increased in the zonefarthest distance from said lens.
 7. The infrared intrusion sensor setforth in claim 1 wherein the concentration of detector elements isroughly inversely proportional to the square of the range.